Precision counters can measure time intervals, pulse widths, and transition times with resolution as fine as 20 picoseconds. Although the counter resolution may be in the tens of picoseconds, the accuracy of the counter results itself is limited by systematic errors. These errors can be one nanosecond or larger. Averaging results reduces resolution even further, for example, 1 to 2 picoseconds. But averaging does nothing for systematic errors, so that accuracy remains limited.
Systematic errors are caused by differences in delay between start and stop channel paths in a counter. For example, uncertainties in the trigger level, differences in the propagation delay in the input amplifiers of the counter, and differences in the electrical path lengths in cables or probes leading to the counter all contribute to systematic delay errors in the counter. Furthermore, these delay errors are typically dependent on the nature of the input signal being measured and may vary with input amplitude, slew rate, trigger level, and slope.
To account for systematic errors in counters, the counters are calibrated and the counter measurement results are corrected with calibration constants derived from the calibration process. This calibration process typically involves first providing a known time interaval to the counter, then making a measurement, and finally determining the difference between the measurement result and the known value to derive a calibration constant for correcting future measurements.
Severe difficulties, however, are encountered in the generation of accurate, precise, and repeatable time intervals to act as a standard for calibration. Any attempt to generate signal edges using active devices is unreliable, since active devices will themselves introduce systematic errors which may vary with all the parameters discussed earlier. Additionally, due to finite transition times, that is, rise and fall times of the edges, timing must be measured at precisely well-defined voltages. This is particularly true of signals with opposing slopes. Conventional wisdom usually dictates measurements to be made at "mid-pulse" points; such measurements then necessitate a definition of what constitutes a "full pulse." The determination of a full pulse itself is no trivial matter. To make a proper determination, additional voltage sampling instruments are required to plot out probability densities. Clearly, this additional process unduly complicates the time interval measurement.
In the prior art, the technique for calibrating a counter for time interval measurements between two edges of signals of the same polarity requires two perfectly identical signals applied simultaneously to the input of the START and STOP channels, viz., channel A and channel B, of the counter. The time interval measured is the calibration constant and is subtracted from the results of all similar measurements made by the counter. The difficulty in this calibration technique is assuring that the signals are identical and that their edges occur simultaneously at the same trigger level, or voltage level where timing is measured.
The prior art technique for calibrating a counter to measure the time interval between a positive-going slope and a negative-going slope is even more elaborate. It requires a perfectly symmetrical test pulse. One method of obtaining such a test pulse uses a time-based crystal oscillator to trigger the output of a pulse generator. This pulse generator itself is adjusted with a spectrum analyzer to minimize all even harmonics. The counter being calibrated measures the time interval between the positive-going slope and the negative-going slope of this test pulse. The time interval measured less the half-period of the test pulse gives the systematic error of the counter and its connectors. The accuracy of this procedure is entirely dependent on the time and voltage symmetry of the test pulse and the accuracy of its frequency. Because the spectrum analyzer has active devices, such as transistors, it can introduce an unknown amount of error into this calibration procedure. A means, then, must be used to determine the exact mid-pulse point of the test pulse for this procedure to be valid.
In accordance with the present invention, a method and apparatus are introduced for calibrating a counter for time interval measurements, pulse width meansurements, and transition time skew measurements. These measurements are accomplished with inexpensive linear passive hardware that introduces a minimum amount of distortion, which can be calibrated out along with other systematic errors from other sources. This method according to the invention does not require highly accurate input reference signals. It assures identity of pulses by its linearity and simultaneity through cross-switching of signals through radio-frequency (rf) relays. Because signals are a.c.-coupled, timing is always measured at zero-volt level for all edges. In this way, half-pulse symmetry about zero-volt level is assured.
A biasing method is further provided to extend the usefulness of the method in accordance with the invention from zero-volt to any arbitrary voltage with the same timing accuracy.
Using the method of the present invention, the systematic errors in a counter are determined and a set of calibration constants for the counter are derived using a signal source and linear passive power-splitters and radio frequency relays. By making a series of test measurements in which the sources of systematic error are tested in different combinations, a set of calibration constants for the counter can be derived, which are then subtracted from future similar measurements to compensate for the systematic error in the counter. These test measurements involve using different signal splitters, cross-switching of signals, and configuring the counter to different measurement modes. Both the test measurements and computation can be done automatically in a few seconds using computer control. The calibration method of the invention is applicable when the counter is used alone, or in conjunction with passive or active probes.